First commit, done on M6 from the PLAN.MD

web/js/engine/rng.js

@@ -0,0 +1,31 @@
+// mulberry32 PRNG. The generator state lives in state.rngState (a uint32) so a
+// saved game resumes with the exact same sequence.
+
+export function nextFloat(state) {
+  state.rngState = (state.rngState + 0x6d2b79f5) >>> 0;
+  let t = state.rngState;
+  t = Math.imul(t ^ (t >>> 15), t | 1);
+  t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
+  return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+}
+
+// Uniform in [-amp, amp].
+export function nextNoise(state, amp) {
+  return (nextFloat(state) * 2 - 1) * amp;
+}
+
+export function nextInt(state, maxExclusive) {
+  return Math.floor(nextFloat(state) * maxExclusive);
+}
+
+export function pickWeighted(state, items, weights) {
+  let total = 0;
+  for (const w of weights) total += w;
+  if (total <= 0) return null;
+  let r = nextFloat(state) * total;
+  for (let i = 0; i < items.length; i++) {
+    r -= weights[i];
+    if (r < 0) return items[i];
+  }
+  return items[items.length - 1];
+}